[BK, DIRTY] = BKBEGIN(BK) Begins a block BK by first checking if
it exists. If the block exists, it checks if any input blocks have
changed since this block was last executed. It also checks if any
input parameters have been changed, added, or removed. If any of
these conditions is true, DIRTY is set and the timestamp on the
block is updated. BK contains the modified block.